Dry friction clutch/brakes depend upon the rubbing of a dry friction material against dry reaction members to start and stop a press. This continuous rubbing causes wearing of both the friction material and the reaction members as well as causing the generation of heat in these members. The faster the press operates and the faster the flywheel rotates, greater is the amount of wear and heat which are generated. This generation of wear and heat requires periodic gap adjustment between the friction plates and the reaction members to keep the press operating correctly. The trip rate or cycle speed of a press equipped with a dry friction clutch/brake unit is limited because the mass of the unit determines its heat dissipation capacity. If the mass of the unit is increased to increase its heat dissipation capacity, the inertia that must be started and stopped is also increased. These factors define a closed loop from which it is impossible to escape when trying to increase the performance of the system.
Oil shear brake and clutch units have been developed to eliminate the problems associated with the dry friction type of units. The oil shear systems use hydraulic actuation instead of air actuation and the units have been developed with key modules which can be easily adapted to be mated with the various types of presses. Properly designed oil shear clutch/brake drives offer the advantage of little or no wear of plates in the disk stacks and no brake fade. This provides a more precise operation of the press and dramatically increases press up-time. The oil film between the adjacent disks carries the heat generated by the start-stops of the press away from the disk stacks. This removal of heat offers the advantage that there is now no practical limit in the press trip rate and the flywheel speed. In addition, this removal of heat provides unlimited inching capabilities.
While these oil shear clutch/brake drive units have significantly improved the operation of the presses, they have all experienced problems associated with the controlled coordination between the operation of the clutch and brake units. One problem associated with these prior art oil shear clutch/brake drives has been the inability to accurately control the amount of overlap between the operation of the brake unit and the operation of the clutch unit.
Some press applications require the operation of the clutch to be completely separate from the operation of the brake, thus eliminating all amounts of overlap between them. This elimination of overlap provides a dead zone in which the clutch is not engaged and the brake is not applied. When providing a dead zone, it becomes more and more difficult to meet the emergency stopping time requirements of the various safety standards because the system must pass through this dead zone to effect stopping of the press.
In addition to the problems related to emergency stopping time requirements, this elimination of overlap may also cause problems when it is desirable to inch through the stroke of the press without experiencing a backing up of the press. This backup of the press can occur as the clutch/brake drive units pass through this dead zone. In these applications, it becomes necessary to provide a certain amount of overlap between the operation of the clutch and the operation of the brake. While these applications require a small amount of overlap, it is imperative to accurately control the amount of overlap in order to maintain the operational life of the clutch/brake drive unit.
Accordingly, what is needed is a press drive system which incorporates the flexibility to be applied to a wider variety of presses where the amount of overlap can be accurately controlled to meet the specific requirements of the application.